The present invention relates to a modulating and demodulating device (hereinafter referred to as MODEM) for data transmission systems, and more particularly to a modem capable of automatically determining whether to set the delay equalizer or not.
A data transmission system links a data processing apparatus with a data input/output apparatus or data processing apparatus together, located in distance from each other, for the transmission of information between them. For this purpose, a data transmission system is composed of various transmission paths and transmitting apparatuses. Data processing apparatuses and data input/output apparatuses are collectively called data terminal equipment (DTE). Data transmitting apparatuses in a data transmission system are known as data circuit terminating equipment (DCE). A MODEM, a typical example of DCE, achieves reciprocal conversion between the signal interfacing conditions between DTE units and those on transmission paths.
Usually, a MODEM in such a data transmission system is provided with a delay equalizer to compensate for delay distortion, and sometimes group delay distortion, to which the transmitted data may suffer on the transmission path. Group delay distortion on a telephone line, used as the transmission path between MODEM's opposite to each other, varies in characteristic with the number of links on the line and other factors. On an exclusive line, the characteristic of group delay distortion is determined when the line is laid, and basically is subject to no major subsequent change. Therefore, the delay equalizer is set by the installing technician at the time of installing the MODEM on the basis of the group delay distortion characteristic of the line which he determines by an actual data transmission test using the MODEM. In this setting procedure, the technician comparatively observes the eye patterns on an oscilloscope or some other precision instrument when the delay equalizer is inserted on the line and when not. Based on the result of this comparative test, the technician judges whether or not the delay equalizer should be set.
However, the above described setting procedure for the delay equalizer requires not only a precision instrument such as an oscilloscope but also a skilled technician for the installation work.